The present invention relates to digital communications using Orthogonal Frequency Multiplexing (OFDM) technique. More particularly, the invention relates to channel estimation in OFDM systems.
Signal processing systems such as communication receivers often must recover a desired signal that has been transmitted through a channel and is degraded under the influence of multipath and the like during the transmission. In order to compensate for the signal impairment introduced thereby, receivers can use signal processing techniques to estimate the channel conditions.
OFDM communications systems are widely used for transmitting digital information. Current OFDM systems are, for example, Digital Audio Broadcasting (DAB), Digital Video Broadcasting such as DVB-Terrestrial (DVB-T), Integrated Services Digital Broadcasting-Terrestrial (ISDB-T), and Wireless Local Area Network (WLAN) such as IEEE 802.11a/b/g/n.
In an OFDM system, a number of subcarriers is independently modulated. The modulation can be Quadrature Amplitude Modulation (QAM) or Phase shift Keying (PSK). The baseband signal in an OFDM system includes multiple OFDM symbols, each OFDM symbol contains a predetermined number of sub-carriers with the majority of the subcarriers designated to carry user data and some subcarriers designated to carry pilot signals. The data sub-carriers are hereinafter referred as “data carriers” and the pilot subcarriers are hereinafter referred as “pilot tones.” Pilot tones are dispersed or scattered among the data carriers within an OFDM symbol. Pilot tones have known frequencies and phase modulation to provide a phase reference for data carriers in the OFDM symbol for improving the accuracy of the signal demodulation at the receiver. Pilot tones are generally spaced apart in frequency by an amount that permits the channel response of carriers lying in-between the pilot tones to be accurately estimated by interpolating the channel responses determined for the pilot tones.
FIG. 2 is a high level block diagram of an OFDM channel estimation device 200, as known in the prior art. OFDM channel estimation device 200 includes a discrete Fourier Transform block 212 that converts a time-domain digital baseband signal D0 to a frequency-domain signal D1 having at least one OFDM symbol. The OFDM symbol is the sum of data carriers and pilot tones. A pilot tone extractor 222 picks out the pilot tones and provides a channel frequency response D2 at its output. FIG. 3 illustrates an exemplary waveform diagram of the extracted pilot tones. An inverse discrete Fourier Transform block 232 converts the channel frequency response D2 to a channel impulse response signal D3. The channel impulse response signal D3 may include a time-domain signal having one or more peaks that represent a more or less direct received signal and multiple delayed signals caused by the multipath. The one or more peaks may have non-negligible energy levels and can be a measure of the delay spread, which is interpreted as the difference between the time of arrival of the first significant multipath component and the time of arrival of the last multipath component. The channel impulse response signal D3 may include other multipath signals whose energy levels are not significant and will be removed in a subsequent noise removal block 242. The noise-reduced channel impulse response D4 is then fed to a second discrete Fourier Transform block 252 that converts the channel impulse response D4 to a final channel frequency response D5 that is representative of the “real” channel characteristic.
FIGS. 3 to 5 illustrate the magnitude spectrum of data outputs at different stages of the OFDM channel estimation device 200. FIG. 3 is a waveform diagram showing the extracted pilot tones from a received OFDM symbol at the output of the pilot extractor block 222. The x-axis is shown in frequency (Hz), and the y-axis is given for a normalized power spectrum density (PSD) in dB.
FIG. 4 is a channel impulse response signal D4 at the output of the noise removal block 242. Channel impulse response signal D4 contains most of the channel impulse energy with much reduced noise level. The x-axis is shown in delay time unit (ns) and the y-axis is given for a PSD in dB.
FIG. 5 is an estimated channel frequency response signal D5 at the output of the second discrete Fourier Transform block 252. The x-axis is shown in frequency (Hz) and the y-axis is shown for a PSD (dB).
FIG. 6 is a waveform showing the error of the estimated channel frequency response against a 0 dB AWGN channel (ideal channel), as known in the prior art.